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An experiment to search for light sterile neutrinos was conducted at a reactor with a thermal power of 2.8 GW located at the Hanbit nuclear power complex. The search was done with a detector consisting of a ton of Gd-loaded liquid scintillator in a tendon gallery approximately 24 m from the reactor core. The measured antineutrino event rate is 1976 per day with a signal to background ratio of about 22. The shape of the antineutrino energy spectrum obtained from eight-month data-taking period is compared with a hypothesis of oscillations due to active-sterile antineutrino mixing. It is found to be consistent with no oscillation. An excess around 5 MeV prompt energy range is observed as seen in existing longer baseline experiments. The parameter space of $sin^{2}2theta_{14}$ down below 0.1 for $Delta m^{2}_{41}$ ranging from 0.2 eV$^{2}$ to 2.3 eV$^{2}$ and the optimum point for the previously reported reactor antineutrino anomaly are excluded with a confidence level higher than 90%.
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The experiment Neutrino-4 had started in 2014 with a detector model and then was continued with a full-scale detector in 2016 - 2021. In this article we describe all steps of preparatory work on this experiment. We present all results of the Neutrino-4 experiment with increased statistical accuracy provided to date. The experimental setup is constructed to measure the flux and spectrum of the reactor antineutrinos as a function of distance to the center of the active zone of the SM-3 reactor (Dimitrovgrad, Russia) in the range of 6 - 12 meters. Using all the collected data, we performed a model-independent analysis to determine the oscillation parameters $Delta m_{14}^2$ and $sin^22theta_{14}$. The method of coherent summation of measurement results allows to directly demonstrate the oscillation effect. We present the analysis of possible systematic errors and the MC model of the experiment, which considers the possibility of the effect manifestation at the present precision level. As a result of the analysis, we can conclude that at currently available statistical accuracy we observe the oscillations at the $2.9sigma$ level with parameters $Delta m_{14}^2=(7.3pm0.13_{st}pm1.16_{sys})text{eV}^2 = (7.3pm1.17)text{eV}^2$ and $sin^22theta_{14}= 0.36pm0.12_{stat}(2.9sigma)$. Monte Carlo based statistical analysis gave estimation of confidence level at $2.7sigma$. We plan to improve the currently working experimental setup and create a completely new setup in order to increase the accuracy of the experiment by 3 times. We also provide a brief analysis of the general experimental situation in the search for sterile neutrinos.
DANSS is a highly segmented 1~m${}^3$ plastic scintillator detector. Its 2500 one meter long scintillator strips have a Gd-loaded reflective cover. The DANSS detector is placed under an industrial 3.1~$mathrm{GW_{th}}$ reactor of the Kalinin Nuclear Power Plant 350~km NW from Moscow. The distance to the core is varied on-line from 10.7~m to 12.7~m. The reactor building provides about 50~m water-equivalent shielding against the cosmic background. DANSS detects almost 5000 $widetilde u_e$ per day at the closest position with the cosmic background less than 3$%$. The inverse beta decay process is used to detect $widetilde u_e$. Sterile neutrinos are searched for assuming the $4 u$ model (3 active and 1 sterile $ u$). The exclusion area in the $Delta m_{14}^2,sin^22theta_{14}$ plane is obtained using a ratio of positron energy spectra collected at different distances. Therefore results do not depend on the shape and normalization of the reactor $widetilde u_e$ spectrum, as well as on the detector efficiency. Results are based on 966 thousand antineutrino events collected at 3 distances from the reactor core. The excluded area covers a wide range of the sterile neutrino parameters up to $sin^22theta_{14}<0.01$ in the most sensitive region.
We present the results of the Neutrino-4 experiment on search for a sterile neutrino. The experiment has been carried out on the SM-3 reactor having a compact active zone of $42times42times35textrm{cm}^3$ and operating on the highly enriched uranium-235 at 90 MW thermal power. We report the results of the Neutrino-4 experiment of measurements of reactor antineutrino flux and spectrum dependence on the distance in the range 6-12 meters from the center of the reactor core. Using the measured spectrum and the distance dependence of antineutrino flux, we performed the model independent analysis of restrictions on the oscillation parameters $Delta m^2_{14}$ and $sin^2 2theta_{14}$. The method of coherent addition of results of measurements is proposed. It allows us to directly observe the effect of oscillations. We observed the oscillation effect at CL $3.5sigma$ in the vicinity of $Delta m^2_{14} approx 7.26textrm{eV}^2$ and $sin^2 2theta_{14} approx 0.38$. Combining the result of the Neutrino-4 experiment and the result of the gallium anomaly effect we obtained value $sin^2 2theta_{14} approx 0.35 pm 0.07 (5sigma)$. The analysis of systematics effects is presented. Comparison with results of other experiments is presented. Future prospect of the experiment is discussed. It is necessary to notice that obtained values $sin^2 2theta_{14} approx 0.35 pm 0.07 (5sigma)$ and $Delta m^2_{14} approx (7.3 pm 0.7)textrm{eV}^2$ allow make assessment on the mass of a neutrino: $m_{beta} approx 0.8textrm{eV}$.
We present a reactor model independent search for sterile neutrino oscillation using 2,509,days of RENO near detector data and 180 days of NEOS data. The reactor related systematic uncertainties are significantly suppressed as both detectors are located at the same reactor complex of Hanbit Nuclear Power Plant. The search is performed by electron antineutrino,($overline{ u}_e$) disappearance between six reactors and two detectors with baselines of 294,m,(RENO) and 24,m,(NEOS). A spectral comparison of the NEOS prompt-energy spectrum with a no-oscillation prediction from the RENO measurement can explore reactor $overline{ u}_e$ oscillations to sterile neutrino. Based on the comparison, we obtain a 95% C.L. excluded region of $0.1<|Delta m_{41}^2|<7$,eV$^2$. We also obtain a 68% C.L. allowed region with the best fit of $|Delta m_{41}^2|=2.41,pm,0.03,$,eV$^2$ and $sin^2 2theta_{14}$=0.08$,pm,$0.03 with a p-value of 8.2%. Comparisons of obtained reactor antineutrino spectra at reactor sources are made among RENO, NEOS, and Daya Bay to find a possible spectral variation.
In the recent years, major milestones in neutrino physics were accomplished at nuclear reactors: the smallest neutrino mixing angle $theta_{13}$ was determined with high precision and the emitted antineutrino spectrum was measured at unprecedented resolution. However, two anomalies, the first one related to the absolute flux and the second one to the spectral shape, have yet to be solved. The flux anomaly is known as the Reactor Antineutrino Anomaly and could be caused by the existence of a light sterile neutrino eigenstate participating in the neutrino oscillation phenomenon. Introducing a sterile state implies the presence of a fourth mass eigenstate, while global fits favour oscillation parameters around $sin^{2}(2theta)=0.09$ and $Delta m^{2}=1.8textrm{eV}^{2}$. The STEREO experiment was built to finally solve this puzzle. It is one of the first running experiments built to search for eV sterile neutrinos and takes data since end of 2016 at ILL Grenoble, France. At a short baseline of 10 metres, it measures the antineutrino flux and spectrum emitted by a compact research reactor. The segmentation of the detector in six target cells allows for independent measurements of the neutrino spectrum at multiple baselines. An active-sterile flavour oscillation could be unambiguously detected, as it distorts the spectral shape of each cells measurement differently. This contribution gives an overview on the STEREO experiment, along with details on the detector design, detection principle and the current status of data analysis.
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